The present invention relates to a charged particle beam exposure system for drawing a pattern on a surface of a substrate by use of a charged particle beam such an electron beam.
In accordance with the high integration of a semiconductor device (LSI) and the enlargement of the capacity, a width of a line to be used in a circuit pattern of the semiconductor device becomes narrower. In the manufacture of such a type of the semiconductor device, there are used several tens kinds of reticles (photomasks) on which various original patterns are formed. After the reticle is positioned to a wafer with high accuracy, the original pattern is transferred to an exposure region of a surface of the wafer. This process is repeated by use of the respective kinds of reticles, sequentially, so that the semiconductor device is manufactured.
For transferring the original pattern formed on the reticle, there is used a reduction projection aligner having a high accurate optical system and a high accurate XY stage. The wafer is set on the XY stage, and the entire surface of the wafer is exposed. This kind of reduction projection aligner is called a stepper since the wafer is exposed against the optical system by step and repeat.
Generally, the original pattern is drawn on a glass substrate finished with high accuracy as follows:
A Cr film is deposited on one surface of the glass substrate, and the resultant surface is uniformly coated with photoresist. The photoresist is irradiated with an electron beam so as to draw a pattern. At the time of drawing the pattern on the photoresist, an electron beam exposure system is used. In the electron beam exposure system, the photoresist is irradiated with a focused electron beam as being scanning over the entire surface of the substrate in accordance with design data. A part of photoresist where the electron beam is irradiated is changed in quality. Then, the above part of photoresist is developed, so that the pattern of photoresist is obtained. In the electron beam exposure system, since there is used the focused electron beam having high resolution, the pattern of photo-resist having high accuracy of the position can be obtained. Then, the above-formed pattern of photoresist is used as an etching mask, and the Cr film is etched. As a result, there can be obtained the original pattern, which is formed of the Cr film.
In the case of exposing the surface of the wafer by use of the original pattern formed on the glass substrate by the stepper, it was considered that it was impossible to set the resolution of the pattern to be less than 1 .mu.m, because of the limitation of the pattern due to the wavelength of light.
However, the pattern resolution of sub-micron can be obtained by improvement of the light source and the optical system, and the use of a phase shift method for adjusting the phase of light on the reticle.
However, since there is still the limitation of improving resolution, the occupying area of the circuit pattern, that is, the size of the semiconductor device must be increased to further improve the enlargement of the capacity of the semiconductor device and the high integration. Due to this, the following point is required at the time of forming the reticle or the photomask. The high accuracy of the position must be maintained over the largest possible area in drawing the original pattern on the glass substrate.
In accordance with fining the width of the line of the circuit pattern, the so-called direct writing method is used. In this method, high resolution of the electron beam is used. The circuit pattern is directly drawn on the wafer by the electron beam without using the original pattern. Even in such a direct writing method, the pattern must be drawn with high accuracy on the largest possible area up to the portion close to the periphery of the wafer in order to effectively use the area of the wafer surface.
In the case of the pattern-writing by use of the electron beam, the electron beam is deflected by the inclination of the electric potential and the arrival position of the beam is shifted if the electric field of the surrounding of the portion where the electron beam is irradiated is not uniform. As a result, the positional accuracy of the pattern is reduced.
The end surface of the glass substrate is not covered with the Cr deposition film. Due to this, if the electron beam is irradiated to a portion close to the periphery of the glass substrate to draw the pattern, the charge-up occurs on the end surface of the substrate by a generating secondary electron. By the occurrence of the charge-up, the electric field becomes ununiformed, and the positional accuracy of the drawing pattern is reduced. To avoid this problem, Japanese Patent Application KOKAI No. 5-347242 proposes a method in which the peripheral portion of the glass substrate is covered with an electrical conductive cover. However, since photoresist covering the Cr film is non-conductive, the electric potential of the surface of photoresist is generated by irradiation of the electron beam. As a result, an electric field is formed by the electric potential difference between the electrical conductive cover and the surface of photoresist. The electric field deflects the electron beam. As a result, there newly occurs a problem in which the positional accuracy of the pattern is reduced in the vicinity of the periphery.